Abstract Detail

Towards a reclassification of Dioscoreales using organellar phylogenomics and morphology .

Despite being a relatively small order with ~865 species, Dioscoreales display a diverse range of vegetative and reproductive morphologies. Its ~24 genera encompass the economically important Dioscorea yams, lineages with reticulate leaf venation, and mycoheterotrophic taxa in which loss of photosynthesis has resulted in highly modified and reduced morphologies. Previous studies have produced a basic phylogenetic framework for the order, but multiple unresolved higher-order relationships persist, and family-level circumscriptions remain in a great deal of flux, apart from Nartheciaceae. The latest Angiosperm Phylogeny Group (APG IV) classification recognized three families (Burmanniaceae, Dioscoreaceae, Nartheciaceae), grouping all mycoheterotrophs under Burmanniaceae. However, they based the latter grouping on analyses that included some problematic sequences, and noted that more data are needed to assess whether Thismiaceae should be differentiated from Burmanniaceae, or Taccaceae from Dioscoreaceae. Here we present results from organellar phylogenomic analyses, together with tree inferences and ancestral-state reconstructions based on morphology. Our genus-level phylogenomic analyses resolve most higher-order relationships with strong support. As currently circumscribed, Dioscoreaceae and Burmanniaceae are both rejected as monophyletic in the plastid and mitochondrial analyses. They instead depict photosynthetic Tacca (Taccaceae) as closely related to mycoheterotrophic Thismiaceae, the latter excluding mycoheterotrophic Afrothismia. Morphology-based phylogenetic inferences, which include several rare genera that have not yet been sequenced, also strongly support separation of Thismiaceae from Burmanniaceae. We reconstructed the evolution of 35 morphological characters on molecular trees and find, for example, that the presence of funnel-shaped stigma lobes and three stamens are synapomorphies of a narrowly defined Burmanniaceae (excluding Thismiaceae). We relate these and other morphological synapomorphies to various family-level classification schemes of Dioscoreales.